The present invention relates to the formation of fused silica and, more particularly, to a method and apparatus for forming fused silica from liquid silicon-containing compounds.
Various processes are known in the art that involve the production of metal oxides from vaporous reactants. Such processes require a feedstock solution, a means of generating and transporting vapors of the feedstock solution (hereafter called vaporous reactants) and an oxidant to a conversion reaction site, and a means of catalyzing oxidation and combustion coincidentally to produce finely divided, spherical aggregates, called soot. This soot can be collected on any deposition receptor in any number of ways ranging from a collection chamber to a rotating mandrel. It may be simultaneously or subsequently heat treated to form a non-porous, transparent, high purity glass article. This process is usually carried out with specialized equipment having a unique arrangement of nozzles and burners.
Much of the initial research that led to the development of such processes focused on the production of bulk fused silica. Selection of the appropriate feedstock was an important aspect of that work. Consequently, it was at that time determined that a material capable of generating a vapor pressure of 200-300 millimeters of mercury (mm Hg) at temperatures below 100xc2x0 C. would be useful for making such bulk fused silica. The high vapor pressure of silicon tetrachloride (SiCl4) suggested its usefulness as a convenient vapor source for soot generation and launched the discovery and use of a series of similar chloride-based feedstocks. This factor, more than any other, is responsible for the presently accepted use of SiCl4, GeCl4, POCl3, and BCl3 as vapor sources, even though these materials have certain chemically undesirable properties.
Silicon, germanium, zirconium, and titanium are metals often used in halide form as vaporous reactants in forming metal oxide glasses. However, SiCl4 has been the industry standard over the years for the production of high purity silica glasses. As disclosed in U.S. Pat. No. 3,698,936, one of several reactions may be employed to produce high purity fused silica via oxidation of SiCl4; namely:
SiCl4+O2xe2x86x92SiO2+2Cl2,xe2x80x83xe2x80x83(1)
SiCl4+⅔O3xe2x86x92SiO2+2Cl2, orxe2x80x83xe2x80x83(2)
SiCl4+2H2Oxe2x86x92SiO2+4HCl,xe2x80x83xe2x80x83(3)
whereby burners or jet assemblies are utilized in feeding the reactant gases and vapors to a reaction space. It should be noted that reaction (2) rarely occurs or is used. There are inherent economic disadvantages to each of these reactions. Moreover, these reactions, which oxidize SiCl4 through pyrolysis and hydrolysis, have the disadvantage of producing chlorine or a very strong acid by-product.
While the first two reactions occur theoretically, an auxiliary fuel is generally needed to achieve pyrolytic temperature. The hydrolysis of SiCl4 results in the formation of hydrochloric acid (HCl), a by-product that is detrimental not only to many deposition substrates and to reaction equipment but also is harmful to the environment. Emission abatement systems have proven to be very expensive due to down-time, loss, and maintenance of equipment caused by the corrosiveness of HCl.
Notwithstanding the problems with handling and disposal of the HCl by-product, the third reaction, hydrolysis of SiCl4, tends to be the preferred commercial method of producing fused silica for economic reasons.
Though hydrolysis of SiCl4 has been the preference of industry for producing high purity fused silica over the years, the enhanced global sensitivity to environmental protection has led to more strict government regulation of point source emissions, prompting a search for less environmentally pernicious feedstocks. Point source emission regulations require that HCl, the by-product of hydrolyzing SiCl4, as well as many particulate pollutants be cleansed from exhaust gases prior to their release into the atmosphere. The economic consequences of meeting these regulations have made commercial production of fused silica from halide-based feedstocks less attractive to industry.
As an alternative, high purity fused quartz or silica has also been produced by thermal decomposition and oxidation of silanes. However, this requires taking safety measures in handling because of the violent reaction that results from the introduction of air into a closed container of silanes. Silanes react with carbon dioxide, nitrous oxide, oxygen, or water to produce high purity materials that are potentially useful in producing, among other things, semiconductor devices. However, silanes have proven to be much too expensive and reactive to be considered for commercial use except possibly for small scale applications requiring extremely high purity.
A number of patents describe the production of high purity metal oxides, particularly fused silica, from a chloride-based feedstock. These patents disclose equipment with a number of burner arrangements and feedstock delivery systems to achieve oxidation of a metal chloride through flame hydrolysis or pyrolysis. Illustrative of this is U.S. Pat. No. 4,491,604 to Lesk et al., where trichlorosilane, dichlorosilane, and silicon tetrachloride are flame hydrolyzed to form soot, and U.S. Pat. No. 3,666,414 to Bayer, where silicon halides such as trichlorosilane or chloroform are flame hydrolyzed. In similar processes, U.S. Pat. Nos. 3,486,913 to Zirngibl (xe2x80x9cZirngiblxe2x80x9d) and 2,269,059 to McLachlan (xe2x80x9cMcLachlanxe2x80x9d) teach oxidation of halides. Volatilized inorganic halide components such as TiCl4, CrCl3, CrO2Cl2, SiCl4, AlCl3, ZrCl4, FeCl2, FeCl3, ZnCl2, or SnCl4 that are oxidized with air, steam, or oxygen are employed in Zirngibl, while silicon halides, ethyl silicate, methyl borate, TiCl4, AlCl3, and ZrCl4 are used by McLachlan.
U.S. Pat. No. 3,416,890 to Best et al. discloses a process for preparing finely-divided metal or metalloid oxides by the decomposition of a metal or metalloid perhalide in a flame produced by the combustion of an oxidizing gas and an auxiliary fuel, such as carbon disulfide, carbon selenide sulfide, thiophosgene, or other hydrogen-free compounds containing sulfur bonded directly to carbon.
U.S. Pat. No. 2,239,551 to Dalton discloses a method of making glass by decomposing a gaseous mixture of glass-forming compounds in a flame of combustible gas. The mixture is used in the formation of anhydrous oxides of silicon, aluminum, and boron. Decomposable compounds such as ethyl or methyl silicate, trichlorosilane, and silicon tetrafluoride may be substituted for silicon tetrachloride; methyl borate or boron hydride may be substituted for boron fluoride, etc.
U.S. Pat. No. 2,326,059 to Nordberg details a technique for making silica-rich ultra-low expansion glass by vaporizing tetrachlorides of Si and Ti into the gas stream of an oxy-gas burner, depositing the resultant mixture to make a preform, vitrifying the preform at 1500xc2x0 C. to make an opal glass, and firing the opal preform at a higher temperature to cause it to become transparent.
U.S. Pat. No. 2,272,342 to Hyde discloses a method of producing glass articles containing vitreous silica by vaporizing a hydrolyzable compound of silicon such as silicon chloride, trichlorosilane, methyl silicate, ethyl silicate, silicon fluoride, or mixtures thereof, using a water bath. The silicon compound vapor is hydrolyzed by water vapor in the flame of a burner, and the resulting amorphous oxide is collected and subsequently sintered until a transparent glass results.
U.S. Pat. No. 4,501,602 to Miller et al. describes the production of particulate metal oxide soot through the vapor phase deposition of xcex2-diketonate complexes of metals from Groups IA, IB, IIA, IIB, IIIA, IIIB, IVA, IVB, and the rare earth series of the Periodic Table.
Also cited in the art are several patents where silane compounds have been used in producing high purity fused silica.
Japanese Pat. Application No. 90838-1985 to Okamoto et al., discloses a method of doping quartz glass by utilizing an ester silane expressed by the general formula R1nSi (OR2)4-n and one or more dopants defined by the formulae Ge(OR3)3, B(OR3)3, and PH3, where R1 is a hydrogen atom, methyl or ethyl group; R2 is a methyl or ethyl group; R3 is an univalent hydrocarbon group; and n is an integer ranging between 0 and 4. A great many organometallic compounds are disclosed, including methyltrimethoxysilane, dimethyldimethoxysilane, trimethylmethoxysilane, tetramethoxysilane, methyltriethoxysilane, and tetraethoxysilane.
U.S. Pat. No. 3,117,838 to Sterling describes a method of producing very pure fused quartz or silica by the combined thermal decomposition and oxidation of silanes where either carbon dioxide, nitrous oxide, or water vapor and a silane are fed into a burner or torch jet, and the flame is allowed to impinge on a carbon substrate upon which silica is deposited.
U.S. Pat. No. 4,810,673 to Freeman discloses a method of synthesizing high quality silicon oxides by chemical vapor deposition of a source gas mixture which includes a halogenated silane component and an oxygen source, namely, dichlorosilane and nitrous oxide.
U.S. Pat. No. 4,242,487 to Hasegawa et al. discloses a method of producing a heat resistant, semi-inorganic compound that is useful as a material for various heat-resistant materials by reacting an organoborosiloxane compound with at least one of the group of aliphatic polyhydric alcohols, aromatic alcohols, phenols, and aromatic carboxylic acids at 250xc2x0 C. to 450xc2x0 C. in an inert atmosphere.
As is clear from the preceding discussion, it is highly desirable for both economic and environmental reasons to find halide-free silicon compounds to replace the silicon halide feedstocks typically used to produce high purity silica glass. Such halide-free starting materials would produce carbon dioxide and water, rather than noxious and corrosive HCl, as by-products of the glass-making process.
U.S. Pat. No. 5,043,002 to Dobbins et al., the disclosure of which is hereby incorporated by reference, discloses the usefulness of polymethylsiloxanes, in particular, polymethylcyclosiloxanes such as hexamethylcyclotrisiloxane, octamethylcyclotetrasiloxane (xe2x80x9cOMCTSxe2x80x9d), and decamethylcyclopentasiloxane in a method of making fused silica glass. The method can be applied to the production of a non-porous body of silica glass doped with various oxide dopants and for the formation of optical waveguide fibers. U.S. Pat. No. 5,043,002 to Dobbins et al. also discloses the use of hexamethyldisiloxane; see also reference to hexamethyldisiloxane in Japanese Patent Application No. 1-138145.
U.S. Pat. No. 5,152,819 to Blackwell et al., the disclosure of which is hereby incorporated by reference, describes additional halide-free silicon compounds, in particular, organosilicon-nitrogen compounds having a basic Si-N-Si structure, siloxasilazones having a basic Si-N-Si-O-Si structure, and mixtures thereof, which may be used to produce high purity fused silica glass without the concomitant generation of corrosive, polluting by-products.
Although use of halide-free silicon compounds as feedstocks for fused silica glass production, as described in U.S. Pat. Nos. 5,043,002 and 5,152,819, avoids the formation of HCl, some problems remain, particularly when the glass is intended for the formation of optical waveguide fibers. Applicants have found that, in the course of delivering a vaporized polyalkylsiloxane feedstock to the burner, high molecular weight species can be deposited as a gel in the line carrying the vaporous reactants to the burner or within the burner itself. This leads to a reduction in the deposition rate of the soot preform that is subsequently consolidated to a blank from which an optical waveguide fiber is drawn. It also leads to imperfections in the blank that will produce defective or unusable optical waveguide fiber from the affected portions of the blank.
The present invention is directed to a method for making fused silica glass. A liquid, preferably halide-free, silicon-containing compound capable of being converted by thermal oxidative decomposition to SiO2 is provided and introduced directly into the flame of a combustion burner, thereby forming finely divided amorphous soot. The amorphous soot is deposited on a receptor surface where, either substantially simultaneously with or subsequently to its deposition, the soot can be consolidated into a body of fused silica glass. The body of fused silica glass can then be either used to make products directly from the fused body, or the fused body can be further treated, e.g., by drawing to make optical waveguide fiber further see, e.g., the end-uses described in the U.S. Pat. Application No. 08/574,961 entitled xe2x80x9cMethod for Purifying Polyalkylsiloxanes and the Resulting Productsxe2x80x9d, the contents of which are hereby incorporated by reference.
The invention further comprises an apparatus for forming fused silica from liquid, preferably halide-free, silicon-containing reactants that comprises: a combustion burner which, in operation, generates a conversion site flame; an injector for supplying a liquid silicon-containing compound to the flame to convert the compound by thermal oxidative decomposition to a finely divided amorphous soot; and a receptor surface positioned with respect to said combustion burner to permit deposition of the soot on the receptor surface.
Forming amorphous fused SiO2 soot particles from a feedstock comprising a volatile silicon-containing compound typically entails vaporization of that compound prior to its introduction into a combustion burner. In the previously mentioned U.S. Pat. No. 5,043,002 to Dobbins et al., for example, a carrier gas such as nitrogen is bubbled through a silicon-containing reactant compound, preferably a halide-free compound such as octamethylcyclotetrasiloxane. A mixture of the reactant compound vapor and nitrogen is transported to the burner at the reaction site, where the reactant is combined with a gaseous fuel/oxygen mixture and combusted.
Although use of halide-free silicon compounds as feedstocks for fused silica glass production, as described in U.S. Pat. Nos. 5,043,002 and 5,152,819, avoids the formation of HCl, some problems remain, particularly when the glass is intended for the formation of high quality optical products such as optical waveguide fibers. Applicants have found, as disclosed in copending U.S. pat. application No. 08/574,961 entitled xe2x80x9cMethod for Purifying Polyalkylsiloxanes and the Resulting Productsxe2x80x9d, that the presence of high boiling point impurities in, for example, a polyalkylsiloxane feedstock, can result in the formation of gel deposits in the vaporization and delivery systems carrying the vaporous reactants to the burner or within the burner itself. Such polymerizing and gelling of the siloxane feedstock inhibits the controllability and consistency of the silica manufacturing process. This problem is aggravated when an oxidizing carrier gas such as oxygen is included in the reactant vapor stream, because oxidizers can catalyze polymerization of the siloxane feedstock. This leads to a reduction in the deposition rate of the soot preform that is subsequently consolidated to a blank from which an optical waveguide fiber is drawn. Furthermore, particulates of the high molecular weight, high boiling impurities may be deposited on the optical waveguide fiber blank, resulting in xe2x80x9cdefectxe2x80x9d or xe2x80x9cclustered defectxe2x80x9d imperfections that adversely affect the quality of the subsequently drawn optical waveguide fiber and can require scrapping of an entire blank.
Defects are small (i.e. 0.1 to 4.0 mm in diameter) bubbles in a glass body. They can be formed in fused silica by an impurity, such as uncombusted gelled polyalkylsiloxane. A very small particle of siloxane gel can be the initiation site for a defect. The siloxane decomposes at high temperature after being deposited on the glass body, giving off gases which cause the formation of the defect.
Thermophoresis is the process by which soot is attracted to the preform. In fact, it produces the driving force which moves the particles towards the cooler preform. The hot gases from the burner pass around the preform during laydown; the soot particles do not have sufficient momentum by combustion alone to strike the preform. Thermophoresis moves particles in a temperature gradient from hot regions to cooler regions. The burnt gases from a burner are hotter than the preform. As these gases pass around the preform, a temperature gradient is produced. Hot gas molecules have higher velocity than cold gas molecules. When hot gas molecules strike a particle, they transmit more momentum to the particle than a cold gas molecule does. Thus, particles are driven towards the colder gas molecules and, in turn, toward the preform.
Clustered defects are larger glass defects found in optical waveguide fiber preforms. They are made up of a series of defects in the form of a line or a funnel- or flower-shaped cluster. A large particle of gel can be the initiation site for a clustered defect. After the gel particle has struck the porous preform, it causes a raised area to stand out from the preform surface. Because the clustered defect is a raised site, more heat transfer passes to this site. Because of this increased heat transfer, more thermophoresis occurs at this site, causing the imperfection to grow and leave behind a string of defects. As a result of the clustered defect, the affected portion of the optical waveguide fiber preform cannot be consolidated normally, and the consequent irregularity in the blank yields defective optical waveguide fiber. In the case of a typical 100 kilometer consolidated blank, which has a diameter of 70 millimeters (mm) and a length of 0.8 meter (m), the presence of one clustered defect on the surface of the blank will typically result in the loss of 5 kilometers of optical waveguide fiber on drawing. In the case of a larger consolidated blank, the negative impact of a single clustered defect is proportionately higher. In a 250 kilometer consolidated blank, which has a diameter of 90 mm and a length of 1.8 m, one clustered defect on the surface of the blank will typically result in the loss of 8 kilometers of optical waveguide fiber on drawing.
The applicants have now discovered that the above-described problem is inhibited by delivering the siloxane feedstock in the liquid form to the conversion site during the silica manufacturing process. By delivering the siloxane feedstock as a liquid instead of as a vapor, gelling of the siloxane feedstock is prevented in that exposure of the siloxane feedstock to the high temperature environments of a vaporizer and vapor delivery system are avoided. This improves the yield and quality of the fused silica produced and also reduces the maintenance requirements of the production system.